1. Field of the Invention
The present invention relates to unmanned aerial vehicles (UAVs), and particularly to a vertical takeoff and landing UAV.
2. Description of the Related Art
A wide variety of aircraft designs for achieving vertical takeoff and landing (VTOL) exist. The addition of VTOL capability, however, typically decreases the overall efficiency of the aircraft, both through added weight as well as by making the aerodynamics of the aircraft less efficient. A typical VTOL aircraft is configured similar to a conventional aircraft, but with an added lift fan in most cases. The lift fan may be in communication with the aircraft engine or may be separately powered. The additional lift fan is commonly used in addition to tilting main propellers or jet nozzles such that the combination of the lift fan and the tilting main propellers or jet nozzles produces a vertical lifting force. Once the aircraft is hovering, the main rotors/nozzles are rotated to face horizontally and the lift fan is turned off, allowing the aircraft to fly in a normal horizontal mode.
Such an aircraft requires the addition of the lift fan and its associated components, which are solely used during takeoff and landing. During horizontal flight, the lift fan and its associated components are essentially dead weight. Further, if the lift fan and its associated components are not stored away in the fuselage, they tend to increase drag on the aircraft.
Another common configuration of VTOL aircraft uses multiple open rotors aligned vertically for take-off and landing, and while in hover mode, the rotors are turned in the horizontal direction (in a so-called “tiltrotor” craft) or tilted a few degrees for forward motion (in a “quadcopter” craft). However, tiltrotors can only takeoff and land vertically, since they use very large propellers whose diameters are larger than the ground clearance. Further, an aircraft with a very large propeller diameter cannot fly at high subsonic speeds due to formation of shock waves over the propeller's blades, which deteriorates its performance. Moreover, when a pair of tiltrotors are used, this configuration is susceptible to instabilities due to misalignment of the thrust vector with the aircraft's center of gravity.
Quadcopters, which are aircraft with four vertically mounted propellers and no fixed wings, rely on the propeller's thrust for vertical flight, as well as for forward motion. Quadcopters offer flexibility and maneuverability, but their lack of a fixed wing reduces their operational range and cruise speed, since a significant amount of their stored energy is spent in lifting the vehicle via thrust force instead of aerodynamic lift. Thus, a vertical takeoff and landing unmanned aerial vehicle solving the aforementioned problems is desired.